1. Field of Invention
The invention relates to a printing ink composition, more specifically relates to a water-based printing ink composition suitable for ink-jet printing.
2. Description of Related Art
Ink-jet printing has been conducted on various ink-jet systems. These system include an electric control system in which electrostatic suction is utilized to eject ink, a pressure pulse system in which mechanical vibration or displacement of ink caused by a piezoelectric element is utilized to eject ink, and a thermal ink-jet system in which pressure produced by bubbles formed and grown by heat is utilized to eject ink. Under these systems, drops of ink are generated, and then, the drops of ink in part or in entirety are attached to printing mediums so as to print images thereon.
In such ink-jet printing, water-based ink is commonly used. The water-based ink comprises a water-based solvent (such as water or a mixture of water and a water-soluble organic solvent) and a coloring material (such as a water-soluble dye or pigment) dissolved or dispersed in the water-based solvent. Ink-jet printing can be conducted with good quality in the long term by using the water-based ink, on the condition that the water-based ink has appropriate properties (such as viscosity, surface tension, conductivity and density), that the properties of the ink do not change by heating so as to not form a precipitate, which causes clogging of a nozzle or an orifice of an ink-jet printer, and that the ink has high water resistance and high light resistance.
Recently, there has been a growing need to perform ink-jet printing with water-based ink on plain paper rather than on printing sheets designed specifically for ink-jet printing. However, conventional water-based ink is prone to spread over the plain paper and causes bleeding of ink where ink of different colors are arranged adjacently (hereinafter referred to as xe2x80x9ccolor bleedingxe2x80x9d). A number of proposals have been made to provide water-based ink that does not cause color bleeding even on plain paper in ink-jet printing. However, the proposed inks fall short of satisfying required image quality.
The invention has been developed to overcome the above-mentioned and other problems. According to an aspect of the invention, there is provided an ink composition comprising water, a coloring agent and at least two types of polyalkylene glycol alkyl ethers, each having at least one oxyalkylene in its polyoxyalkylene group, wherein at least one of the polyalkylene glycol alkyl ethers is polypropylene glycol n-butyl ether having three or more oxypropylenes in the molecule. It becomes possible to inhibit color bleeding when ink-jet printing with the ink composition of the present invention.
As used herein, the prefix xe2x80x9cpolyxe2x80x9d is used to denote compounds having one or more oxyalkylenes. Thus, unless otherwise indicated, the polyalkylene glycol alkyl ethers described herein include monooxyalkylene glycol alkyl ethers, such as ethylene glycol n-butyl ether; dioxyalkylene glycol alkyl ethers, such as diethylene glycol n-butyl ether; and polyoxyalkylene glycols having three or more oxyalkylenes.
A printing ink composition of the invention comprises water, a coloring agent, and at least two types of polyalkylene glycol alkyl ethers, each having at least one oxyalkylene in a polyoxyalkylene group, where at least one of the polyalkylene glycol alkyl ethers is polypropylene glycol n-butyl ether having at least three oxypropylenes in the molecule. Hereinafter, for brevity, the polyalkylene glycol alkyl ether having at least one oxyalkylene in the polyoxyalkylene group is referred to as xe2x80x9cpolyalkylene glycol alkyl ether,xe2x80x9d whereas the polypropylene glycol n-butyl ether having at least three oxypropylenes in the molecule is referred to as xe2x80x9cpolypropylene glycol n-butyl ether.xe2x80x9d
The coloring agent is used in an amount of 0.1 to 20% by weight, preferably 0.3 to 15% by weight and more preferably 0.5 to 10% by weight, based on the total weight of the printing ink composition. As the coloring agent, any dyes and pigments can be used alone or in combination. In particular, it is preferable to use water-soluble dyes including direct dyes, acid dyes, basic dyes and reactive dyes, among the dyes. It is also preferable to use inorganic pigments and organic pigments.
Representative examples of water-soluble dyes useful in the invention are C.I. Direct Black 17, 19, 32, 51, 71, 108, 146, 154 and 168, C.I. Direct Blue 6, 22, 25, 71, 86, 90, 106 and 199, C.I. Direct Red 1, 4, 17, 28, 80, 83 and 227, C.I. Direct Yellow 12, 24, 26, 86, 98, 132 and 142, C.I. Direct Orange 34, 39, 44, 46 and 60, C.I. Direct Violet 47 and 48, C.I. Direct Brown 109, C.1. Direct Green 59, C.I. Acid Black 2, 7, 24, 26, 31, 52, 63, 112 and 118, C.I. Acid Blue 9, 22, 40, 59, 93, 102, 104, 113, 117, 120, 167, 229 and 234, C.I. Acid Red 1, 6, 32, 37, 51, 52, 80, 85, 87, 92, 94, 115, 181, 256, 289, 315 and 317, C.I. Acid Yellow II, 17, 23, 25, 29, 42, 61 and 71, C.I. Acid Orange 7 and 19, C.I. Acid Violet 49, C.I. Basic Black 2, C.I. Basic Blue 1, 3, 5, 7, 9, 24, 25, 26, 28 and 29, C.I. Basic Red 1, 2, 9, 12, 13, 14 and 37, C.I. Basic Violet 7, 14 and 27, and C.I. Food Black 1 and 2.
The above-mentioned dyes are suitable for ink-jet printing and offer good performance with great clarity, high water solubility, high stability and high light resistance. However, the dyes useful in the present invention are not limited to the above.
Representative examples of inorganic pigments useful in the invention are carbon black, titanium oxides, and iron oxides.
Representative examples of organic pigments are azo pigments (such as azo lakes, insoluble azo pigments, condensed azo pigments, and chelate azo pigments), polycyclic pigments (such as phtalocyanine pigments, perylenes and perylene pigments, anthraquinone pigments, quinacridone pigments, dioxazine pigments, thioindigo pigments, isoindolinone pigments, and quinophthalone pigments), lake pigments (such as base dye lakes, and acid dye lakes), nitro pigments, nitroso pigments, and aniline black daylight fluorescent pigments.
It should also be noted that the pigment useful in the invention is not limited to the above. For example, it is possible to use any other pigment capable of being dispersed in a water phase, or to use pigments of which the surface has been treated with a surfactant or a polymeric dispersing agent (such as graphite carbon).
In the case of using pigments as the coloring agent, the printing ink composition may be provided by dispersing the pigment in a mixture of the purified water and the polyalkylene glycol alkyl ethers with a dispersing agent according to a conventional method, for example, disclosed in Japanese Laid-Open Patent Publication No. 62-101672, which is herein incorporated by reference. Generally, 0.01 to 20% by weight of the dispersing agent is added into the mixture, based on the total weight of the printing ink composition. Other additives may be added, if necessary.
Typical examples of dispersing agents are polymeric dispersing agents or surfactants as disclosed in Japanese Laid-Open Patent Publication No. 62-101672. Any combination of the polymeric dispersing agents and/or the surfactants as well as one of them alone can be used.
Representative examples of polymeric dispersing agents useful in the invention are gelatin, proteins (such as albumin), natural gum materials (such as gum arabic and gum tragacanth), glucosides (such as saponin), cellulose derivatives (such as methyl cellulose, carboxycellulose and hydroxymethyl cellulose), natural polymers (such as lignosulfonates and shellac), anionic polymers (such as polyacrylates, salts of styrene/acrylate copolymers, salts of vinylnapthalene/acrylate copolymers, salts of styrene/maleate copolymers, salts of vinylnapthalene/maleate copolymers, salts of b-napthalene sulfonic formalin condensates and phosphates), and non-ionic polymers (such as polyvinyl alcohol, polyvinyl pyrrolidone and polyethylene glycol).
Representative examples of surfactants useful in the invention are anionic surfactants (such as fatty alcohol sulfates and alkylarylsulfonates), and non-ionic surfactants (such as polyoxyethylene alkyl ethers, polyoxyethylene alkyl esters, sorbitan alkyl esters and polyoxyethylene sorbitan alkyl esters).
The dispersion can be conducted by using any conventional dispersing machine, such as a ball mill, a roll mill and a sand mill, preferably by using a high-speed sand mill.
According to the invention, the polyalkylene glycol alkyl ethers used in the ink composition are considered particularly important to inhibit color bleeding. It is preferable to use polyalkylene glycol alkyl ethers having five or fewer carbons in its alkyl groups and twelve or fewer carbons in its oxyalkylene groups, respectively, among various polyalkylene glycol alkyl ethers, for the reason that polyalkylene glycol alkyl ethers of relatively long molecular chains have high viscosity and are generally not suitable for ink-jet printing. Particularly, it is essential that at least one of the polyalkylene glycol alkyl ethers is polypropylene glycol n-butyl ether.
The polyalkylene glycol alkyl ethers, other than the polypropylene glycol n-butyl ether, useable in the invention are typified by polyethylene glycol alkyl ethers and polypropylene glycol alkyl ethers.
Representative examples of the polyethylene glycol alkyl ethers useful in the invention are ethylene glycol methyl ether, ethylene glycol ethyl ether, ethylene glycol n-propyl ether, ethylene glycol isopropyl ether, ethylene glycol n-butyl ether, ethylene glycol isobutyl ether, diethylene glycol methyl ether, diethylene glycol ethyl ether, diethylene glycol n-propyl ether, diethylene glycol isopropyl ether, diethylene glycol n-butyl ether, diethylene glycol isobutyl ether, triethylene glycol methyl ether, triethylene glycol ethyl ether, triethylene glycol n-propyl ether, triethylene glycol isopropyl ether, triethylene glycol n-butyl ether and triethylene glycol isobutyl ether.
Representative examples of the polypropylene glycol alkyl ethers useful in the invention are propylene glycol methyl ether, propylene glycol ethyl ether, propylene glycol n-propyl ether, propylene glycol isopropyl ether, propylene glycol n-butyl ether, propylene glycol isobutyl ether, dipropylene glycol methyl ether, dipropylene glycol ethyl ether, dipropylene glycol n-propyl ether, dipropylene glycol isopropyl ether, dipropylene glycol n-butyl ether, dipropylene glycol isobutyl ether, tripropylene glycol methyl ether, tripropylene glycol ethyl ether, tripropylene glycol n-propyl ether, and tripropylene glycol isopropyl ether.
Representative examples of the polypropylene glycol n-butyl ether are tripropylene glycol n-butyl ether, tetrapropylene glycol n-butyl ether, and pentapropylene glycol n-butyl ether.
It is considered that the effect of inhibiting color bleeding is produced as follows in the printing ink composition of the invention. Generally, color bleeding occurs in a printing ink composition that is slow to permeate through a printing medium and is slow to dry. In other words, color bleeding does not occur, unless ink compositions of different colors blend while permeating through the printing medium and before drying. Any of the above-mentioned polyalkylene glycol alkyl ethers, which is not polypropylene glycol n-butyl ether, is dispersed uniformly in the printing ink composition, and improves the ability of the printing ink composition to permeate through a printing medium and to dry slightly at a uniform rate through the ink composition. In addition, the polypropylene glycol n-butyl ether is more likely to be localized at an interface between the ink composition and the air, and increases an initial permeation rate of the ink composition at the time when the ink composition is attached to the printing medium. As a result, the combination of the polyalkylene glycol alkyl ether and the polypropylene glycol n-butyl ether, due to their structural similarity, helps drops of the ink composition permeate from their surface to their core into the printing medium continuously and smoothly. It becomes therefore possible to inhibit color bleeding effectively in the printing ink composition according to the invention.
As described above, inhibiting color bleeding is not as effective when one type of polyalkylene glycol alkyl ether or polypropylene glycol n-butyl ether is used alone in the ink composition. If at least two types of polyalkylene glycol alkyl ethers are used, but all of them are polypropylene glycol n-butyl ethers, color bleeding may be reduced slightly. However, it is more effective to inhibit color bleeding even on plain paper when the polypropylene glycol n-butyl ether is used in combination with another polyalkylene glycol alkyl ether, which is not polypropylene glycol n-butyl ether.
Moreover, color bleeding can not be prevented, even if any conventional surfactant is used in place of the polypropylene glycol n-butyl ether in the ink composition. There are some conceivable reasons for this. One is that the polypropylene glycol n-butyl ether has a smaller molecular size and is capable of moving to the interface between the ink composition and the air faster than conventional surfactant. Another is that, as already mentioned above, the polypropylene glycol n-butyl ether has structural similarity to the other polyalkylene glycol alkyl ether so that the ink composition permeates into the printing medium continuously and smoothly.
The polyalkylene glycol alkyl ethers are used in total in an amount of 2 to 15% by weight, preferably 3 to 12% by weight and more preferably 4 to 10% by weight, based on the total weight of the printing ink composition. If the printing ink composition comprises less than 2% by weight of the polyalkylene glycol alkyl ethers, the ink composition is slow to permeate through the printing medium and to dry, which results in color bleeding. On the other hand, if the printing ink composition comprises more than 15% by weight of the polyalkylene glycol alkyl ethers, the ink composition permeates through the printing medium too much. This presents problems in that the ink composition reaches the back side of the printing medium and bleeding of the ink composition along fibers of the printing medium (which is called xe2x80x9cfeatheringxe2x80x9d) is caused.
Further, a ratio between the polyalkylene glycol alkyl ethers and the polypropylene glycol n-butyl ether is determined in view of a formula and any desired properties of the printing ink composition. Generally, the polyalkylene glycol alkyl ethers except for any polypropylene glycol n-butyl ether is used in an amount of 1 to 80% by weight, preferably 3 to 60% by weight and more preferably 5 to 50% by weight, based on the total weight of the polyalkylene glycol alkyl ethers including the polypropylene glycol n-butyl ether.
As the water, water of high purity (such as ion-exchanged water and distilled water) should be used. In other words, non-purified water (such as tap water, well water and natural water) should not be used. The content of the purified water is determined in view of the types and contents of the coloring agent and the polyalkylene glycol alkyl ether and any desired properties of the printing ink composition. In general, the printing ink composition comprises 10 to 98% by weight, preferably 30 to 97% by weight and more preferably 40 to 95% by weight of the purified water, based on the total weight of the printing ink composition.
The printing ink composition of the invention may further comprises other additives, such as dispersing agents, surfactants, viscosity adjusting agents, surface tension adjusting agents, pH adjusting agents, antiseptics and fungicides, as necessary.
For example, an agent that promotes liquid stability of the printing ink composition (hereinafter referred to as a liquid stabilizer) may be added to prevent the printing ink composition from drying at the nozzles of ink-jet printers. Representative examples of liquid stabilizers useful in the invention are polyalcohols (such as ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, dipropylene glycol, tripropylene glycol, polypropylene glycol, 1,3-butanediol, 1,5-pentanediol, 1,6-hexanediol, glycerin, 1,2,6-hexanetriol, 1,2,4-butanetriol, 1,2,3-butanetriol and 1,3,5-pentanetriol), nitrogen-containing heterocyclic compounds (such as N-methyl-2-pyrrolidone, N-hydroxyethyl-2-pyrrolidone, 2-pyrrolidone, 1,3-dimethyl imidazolidinone and caprolactam), amides (such as formamide, N-methylformamide and N,N-dimethylformamide), amines (such as monoethanolamine, diethanolamine, triethanolamine, monoethylamine, diethylamine and triethylamine), and sulfur-containing compounds (such as dimethyl sulfoxide, sulfolane and thiodiethanol). Any combination of the above-mentioned liquid stabilizers as well as one of them may be chosen depending on particular needs. The content of the liquid stabilizer is determined in view of components and any desired properties of the printing ink composition. In general, the printing ink composition comprises 0 to 40% by weight, preferably 5 to 30% by weight, of the liquid stabilizer, based on the total weight of the printing ink composition.
In the case of a printing ink composition that is electrically charged in order to be ejected from an ink-jet printer, it is generally necessary to add a resistance adjusting agent into the ink composition. Typical examples of the resistance adjusting agent are inorganic salts, such as lithium chloride, ammonium chloride and sodium chloride.
Further, in the case of a printing ink composition that is ejected under the influence of heat, it is generally necessary to select each component of the ink composition and its content, so as to adjust thermal properties (such as resistivity, coefficient of thermal expansion, and coefficient of thermal conductivity) appropriately.
The printing ink composition of the invention, given as described above, provides solutions to problems in known compositions, and ensures ink-jet printing of good image quality with no color bleeding.
The invention will be specifically illustrated in more detail by way of the following synthetic examples and the following comparative examples.